Sulfuric acid plants produce a prodigious amount of high-level waste heat but nearly all of the high-level waste heat is utilized in the production of electricity through a steam turbo/generator.
An alternative to generating power in steam turbines is to expand the hot combustion product gases directly to produce work in a turbine expander. From an exergetic viewpoint this is a more efficient way of utilizing the heat for power production.
Very few examples exist of an expansion turbine used for direct expansion of a reactor product gas for recovery of the reaction heat. In such a system, the turbine expander may impact significantly the downstream operations, disturb optimum process conditions or even require a change in the process configuration. The patent of Janssen et al. (EP 0753652) describes a process for the synthesis of ethane from methane. The exothermic reaction takes place over a catalyst in the combustion chamber of a gas turbine. The reaction products are expanded and the turbine drives the methane and combustion air compressor. The cycle is open with no recycling of reaction products. More recently Agee et al. (U.S. Pat. No. 6,155,039) patented a synthesis gas production system comprising a gas turbine with an autothermal reformer between the compressor and expander. The reformer uses a combination of partial oxidation and steam reforming. The exothermic heat of the partial step provides the heat for an endothermic steam-reforming reaction. The reformer produces synthesis gas and serves as the combustor for the gas turbine.
In relation to energy recovery in sulfuric acid manufacture, conceptual case studies on the subject have been published, namely a study on: turbine expander integration with a sulfuric acid plant by Harman et al. (Gas turbine topping for increased energy recovery in sulfuric acid manufacture, Applied Energy (3) (1977) Applied Science Publisher Ltd, England, 1977).
Harman's study revealed a potential significant advantage in the energy recovery and, for a typical case, the net energy recovery as electric power can be improved by 60-70 percent over that possible with simple steam-rising equipment. For the purpose of modeling a gas turbine plant, Harman chose as a basis a Rolls-Royce ‘Tyne’ engine which has an air flow closely matched to a 600 t/d acid plant. Although burning all the volume of sulfur required by the acid plant was prevented by the maximum temperature constraints (metallurgical limit) of the turbine, he stated that in the event of it being possible to operate a gas turbine at a turbine inlet temperature of around 1400-1500° C., it would become possible to use a turbine as the major energy extraction device for the production of power.
Whether or not this potential advantage can be realized depends on a number of factors including fuel feed and combustion chamber design. In contrast to other liquid fuels, sulfur does not have a light fraction and has a rather high boiling point (450° C.). It has a greater heat of evaporation, surface tension, ignition temperature, and specific gravity, but a lower heat of combustion than, for example fuel oil. Sulfur also loses out to fuel oil with regards to conditions for spraying because of the low-pressure drop across the spray jet due to its higher viscosity and surface tension.
At the sulfuric acid plants, liquid sulfur is burned in dry air in a large refractory-lined chamber before being passed through the waste heat boiler. An important role in sulfur combustion is played by the difference between the boiling point and the temperature at which it is supplied to the furnace. Liquid sulfur is usually sprayed into furnaces at the temperature of minimum viscosity (150° C.) in the form of fine droplets through a variety of nozzles, mainly using pressure atomization. The droplets vaporize at temperatures above the boiling point of sulfur by taking heat from the gases surrounding them and from radiation or convection. The use of greater preheating is held back by the fact that the maximum sulfur viscosity lies between 150 and 400° C., while at temperatures around 160° C. sulfur gets so viscous that it cannot flow, it is deposited on the walls of the sulfur heater, and hinders heat exchange. Thus, the only real source of heat for the initial zone in the process must be the zone of sulfur combustion, whose heat is transmitted by radiation or convection in the organized recirculation of hot combustion products. The flame temperature is, typically 750° C. to 1200° C. depending on the percentage of SO2 required.
The sulfur vapor consists of all molecules from S2 to S8 in temperature- and pressure-dependent equilibria but only the S2 molecules in the vapor phase are actually oxidized. Evaporation of liquid sulfur initially produces mainly S8 molecules, this dissociation proceeds, however, very slowly, which means that sulfur enters the vapor phase also mainly in the form of S8 molecules and the S8 molecules are only decomposed to S2 to any appreciable extent at temperatures above 600° C. More than 60% of the heat reaction (about 9,400 kJ/kg S) liberated in the combustion of sulfur to sulfur dioxide is theoretically required for preheating the air and sulfur and for evaporation and decomposition of the sulfur at 600° C.
Harman at al. in his study examined technical factors such as liquid sulfur feed and combustion chamber design and corrosion resistance of turbine materials to an atmosphere of sulfur dioxide, oxygen and nitrogen.
Harman's study concludes that the combustion system of a gas turbine engine appears capable of burning liquid sulfur, the major modification necessary being temperature control of the plumbing and spray nozzles. This obviates the possible need for a separate refractory-lined burner, fed by the engine compressor and exhausting to the turbine, which would be a potentially very hazardous pressure vessel and would add serious complication and expense to the gas turbine engine installation.
The equipment chosen by Hartman required to vaporize sulfur by the plumbing at high temperature (930 K in case of “Tyne”) which would be needed to maintain it as a vapor at a required pressure (12 atm), means of temperature control by steam jacketing. The sulfur may not be able to ignite from cold and a reasonable starting procedure will be also required. Additional, the nozzles would need to be preheated for the sulfur flow and the exhaust gases generate by preheating by hydrocarbon fuel should be ducted to atmosphere until just before the sulfur combustion is started. The hydrocarbon and sulfur fuels should not be mixed but may well use the existing separate pilot and main nozzle plumbing and orifices, respectively.
The dual fuel capability of the engine would require two high quality control systems to supply the fuels correctly. The engine would need its supply of kerosene at all times to operate the compressor bleed control unless a satisfactory alternative system was provided as an aid to starting. The plant air ducting would need to include an engine bypass and sufficient valving to permit a safe start-up procedure. The existing air blowers would be bypassed when the engine is on line, with a corresponding saving of power. An additional control room may be required.
In addition to Herman's conceptual study, a series of patents by Moichi. JP Pat. No., 60191007, 60191008, 60191009, 60221306, and 60221307 discloses various arrangements of combined pressurized and ordinary-pressure sulfur furnaces but by doing so Moichi added serious vulnerability, complication and expense to the gas turbine engine installation.
Although the prior art proposes some basic techniques for energy recovery in the context of sulfuric acid production, improvements to these prior-art technologies remain highly desirable.